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Old 16-06-2018, 04:37 PM
Startrek (Martin)
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Variations in focusing summer and winter

I was out on Thursday night trying to image NGC6121 with my DSLR on live view and noticed straight away that I had to wind the focus out quarter to half a turn from the bottom in order to achieve good focus on the stars. Whereas during the warmer months I usually image various DSO’s ( 3 to 6 magnitude objects ) with the focuser wound right in as far as it can go for good focus.
I’m assuming the reason for this is the colder air temperatures ( of late between 8deg to 14deg ) contracting the metal of the focuser ,optical tube and brackets etc.. which may
change the focal length in the telescope
Is this a reasonable explanation of the change in focus ?

Appreciate any advice on the observation
Thanks in advance
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Old 16-06-2018, 06:10 PM
raymo
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Expansion and contraction are the cause of the focal changes, but the
effect is nowhere near as great as you describe, so something else
must have changed. Your focuser shouldn't have to be racked right in
to obtain focus. Have you got a Newtonian that has been modified to
make it suitable for imaging or something? I just wonder, because many Newts
don't have enough back focus for imaging. Skywatcher ones are fine, but many GSO
ones need to be modded.[ unless sold as astrographs].
raymo

Last edited by raymo; 16-06-2018 at 06:24 PM. Reason: more text
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Old 16-06-2018, 06:51 PM
Startrek (Martin)
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Hi Raymo,
My scope is a Bintel GSO 6” f6 Newtonian reflector ( stock no mods )
I have been imaging since February at prime focus with focuser racked fully in to achieve good star focus. I only noticed the other night that the focuser (Crayford dual speed ) had to be tweaked out a bit ( probably not half a turn but maybe quarter to eight turn ) to achieve focus. It was noticeable because I haven’t imaged in cold weather yet.We have had cloudy wet conditions in Sydney since end of May. I have not changed anything in my kit at all.

When you do things methodically every time you set and image you do notice any change ( even slight changes )

Martin
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Old 16-06-2018, 07:57 PM
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Atmos (Colin)
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When you collimate the primary, do you always turn the knobs in the same way.
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Old 16-06-2018, 08:43 PM
Startrek (Martin)
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I use a laser collimator and tweak all adjusters on the primary clockwise and anticlockwise as required.I don’t take much notice specifically in the order of adjustment as long as the centre bullseye hole is black with an even red glow around it. I’m pretty sure I don’t use the same set of knobs adjusting the same way every time.
It only takes me 5 minutes to collimate this little scope ( very easy compared to my 10” dob , it can be temperamental some times )
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Old 16-06-2018, 09:52 PM
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Atmos (Colin)
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The primary collimation knobs work by lifting and dropping the mirror al if you were to keep tightening them it would physically push the focus back. It’s the only way I can think of pushing the focus point back so far.
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Old 16-06-2018, 10:02 PM
raymo
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As I said, something is wrong if you have to rack the focuser fully in,
regardless of the ambient temperature. Its analogous to the focus
adjuster on a lens going beyond infinity to allow for expansion or contraction of the lens. I've never seen a focuser that comes to focus at less than 5 or 6mm from from fully in. My newt focuses at 12mm racked out.
raymo

Last edited by raymo; 16-06-2018 at 10:06 PM. Reason: more text
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Old 17-06-2018, 07:10 AM
Startrek (Martin)
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Hi Raymo,

I went out last night and tried to image the jewel box. Used the handcontroller to slew to it , centred it through my illuminated finder , switched on the DSLR with live view at x10 , focuser was racked hard in , noticed stars slightly like donuts, racked out a bit ( eighth of a turn or less ) and achieved good focus at x10 , switched DSLR back to normal view and had pin point sharp stars

Clouds came over just after that so could snap some images

When I image the moon on prime focus , the focuser is racked hard in just to achieve focus ( not even half a mm to spare ) obviously if I use my powermates I have to rack out plus I have plenty of focus adjustment

When I view through any eye piece I have to use a 35mm extension adapter to achieve focus

My scope is an f6 by the way

Cheers Martin
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Old 17-06-2018, 11:58 AM
raymo
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This is a trap many newbies fall into. The Bintel 6" f/6 [like many Newtonians] is optimised for visual work, meaning that it has barely sufficient, or even
insufficient back focus for mounting a camera. As yours is marginal, you are lucky that you can raise the primary mirror up the tube enough [say 5-6mm] by screwing the collimation screws a bit further in.{use longer screws if necessary]. This will allow you to rack backwards and forwards
through focus in order to get precise focus. You will, of course, have to
recollimate the scope after moving the mirror up the tube. If you turn each screw the same amount, the collimation should not be disrupted too much.
raymo
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Old 17-06-2018, 04:14 PM
Startrek (Martin)
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Yes I am lucky this scope can be used to image with a DSLR

When I purchased this scope together with the HEQ5 mount my intention was to use it mainly for visual and then dabble in some planetary and lunar imaging Once I started imaging with the DSLR the scope became more of an imaging scope than visual scope which is the opposite to my original intentions.

Sometime next year I would like to upgrade to an 8” newt and still use my existing 8 year old Canon 600D DSLR which is providing good images ( better than ever expected )

Would you recommend the new Skywatcher black diamond photo reflector 8” f5 as an upgrade from my existing 6” ?
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Old 17-06-2018, 07:13 PM
raymo
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Yes its a good scope. F/5 is easier to work with than f/4, which some
people choose to use. You will need an SW f/5 coma corrector. I suggest
using the SW one because you just install it and you're done; no messing about with spacers.
raymo
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Old 17-06-2018, 07:27 PM
Startrek (Martin)
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Thanks for your advice on the 8”

Cheers
Martin
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